1. Field of the Invention
The present invention relates generally to the field of circulating fluidized bed (CFB) reactors or boilers such as those used in industrial or electric power generation facilities and, in particular, to a primary oxidant feed designed to provide fluidization of bed material through the floor of the CFB.
2. Description of the Related Art
A combustion process where a mix of recycle gas and oxygen is utilized instead of air for firing fossil fuels (also known as oxy-combustion or oxy-firing) is gaining attention as a means for alleviating global warming by reducing the release of green house gases, particularly carbon dioxide (CO2). Replacing the nitrogen contained in the air with the recycle gas results in a higher concentration of CO2 in the flue gas that in turn reduces the cost of sequestering CO2 for its storage underground or underwater.
In the field of oxy-combustion, a circulating fluidized bed (CFB) boiler presents certain advantages over a pulverized coal (PC) boiler. Since the bulk of the matter to be heated in a CFB is represented by the circulating bed material, its “normal” furnace temperature in the case of oxy-combustion can be accomplished with reduced gas recirculation (i.e., higher oxygen concentration in the oxidant streams) compared to PC units. This results in a reduced volume flow of the flue gas, which in turn results in savings on unit size and power consumption.
A typical air-fired CFB furnace utilizes two air streams: primary air and secondary air. Primary air, supplied through the furnace floor, provides fluidization of the bed material and supports combustion in the lower part of the furnace. Secondary air is typically supplied through rows of nozzles located opposite to each other at a certain elevation (in most cases between 10 and 20 feet) above the furnace floor. In the case of oxy-combustion, which typically implies using instead of air an oxidizing agent with increased oxygen concentration, typically comprised predominantly of oxygen and recycled flue gas, the terms “primary air” and “secondary air” should correspondingly be substituted with the terms “primary oxidant” and “secondary oxidant.”
Along with its advantages, oxy-combustion in a CFB presents certain safety issues resulting from a combination of hot solids (potentially backsifting into the furnace windbox) and the high-oxygen stream fed through the windbox. Since starting up and shutting down an oxy-combustion CFB must be done in the conventional (i.e. air combustion) mode, transferring between air-combustion and oxy-combustion firing modes must be addressed.